Amplifier



Patented Aug. 31, 1937 PATENT OFFICE AMPLIFIER John Evans, Riverside, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1933, Serial No. 704,413

7 Claims.

My invention relates to amplifiers and it has particular relation to amplifying stages .in ultrahigh frequency transmitters.

It is well known to those skilled in the art that tuned push-pull amplifying stages are prone to oscillate at the tuned frequency. Various methods have been used in the past to eliminate such spurious oscillations, among which may be cited, as an example, the cross-neutralization 10 system exemplified by the Ballentine Patent No.

A number of other Well known neutralization methods are satisfactory for frequencies of the order of those utilized in radio broadcasting, and

for moderately high frequencies, but such methods are substantially useless at frequencies in the vicinity of 300,000,000 cycles per second, principally by reason of the stray capacity to ground of various potential-supply conductors etc., and also on account of the fact that the neutralizing condensers themselves cause serious losses at ultra-high frequencies.

It is, accordingly, an object of my invention to provide an amplifier of the push-pull type that shall be free from any tendency to oscillate.

Itis a further object of my invention to provide a non-oscillating amplifier stage of the push-pull type that shall be devoid of neutralizing condensers, resistors, and the like commonly utilized for the purpose of preventing oscillation.

t is well known that, to permit oscillations in a thermionic tube, by reasonof the feed-back through the interelectrode capacity, a certain phase relation must exist between the potentials appearing upon the anode with respect to the grid potentials and the potential of the cathode.

Accordingly, in practicing my invention I provide'means whereby the oscillation-favoring potential relationship between the electrodes of the 40 tube is disturbed. Specifically, I prefer to interconnect the cathodes of the several tubes through a tuned circuit and to supply filament potential to the tubes over the said circuit in a novel manner'hereinafter described in detail.

The novel features that I consider characteristic of my invention are set forth with particularly in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing, in which Fig.

55 1 is a semi-diagrammatic view of an amplifier stage including a preferred embodiment of my invention and Fig. 2 is a simplified view of an amplifier stage including my invention, wherein the thermionic devices are of the equi-potential cathode type.

Referring to Fig. 1 of the drawing, an ultra high frequency push-pull amplifier stage including a preferred embodiment of my invention comprises a plurality of thermionic devices l and 3, each of the said devices having an anode ii, a cathode l, and a control grid 9.

Since the amplifier is to be used for ultra-high frequency oscillations of the order of 300,000,000. cycles per second, it is not necessary to provide inductors of usual types for the plate and grid circuits, respectively. For example, the output or plate circuit may be constituted by a plurality of straight conductors II and I3 extending outwardly from the anodes of the tubes, the eifective value of the inductance thereof being determined by a movable bridge member l5, through which plate potential is supplied to the tubes from any convenient source, exemplified in the drawing by the battery I6. A tuning condenser Il may or may not be connected across the conductors H and I3. The antenna may be 7 connected to the output inductors at suitable potential points thereon as indicated by the leads E0 and I9.

The grid or input circuit may also be constituted by a plurality of straight conductors 2| and 23. A tuning condenser 25 may or may not be connected across the conductors 2| and 23 adjacent the grids and between them may be disposed a movable bridging member 21, the position of which determines the effective inductance of the grid circuit.

The filaments of the tubes are connected in parallel to a source 29 of filament potential. For this purpose, it is obvious that each filament re quires two leads and, if the said leads are to be included in a single tuned circuit in order that the objects of my invention may be accomplished, it is necessary that means be provided whereby the said leads, insofar as radio frequencies are concerned, shall function as a single conductor. For this purpose, I provide a plurality of copper tubes 3!, one thermionic device, to which analogous filament terminals are respectively connected and between which extends a movable bridging member 33 that, in turn, is connected to one terminal of the filament potential source 29. The other terminals of the filaments are connected, respectively, to insulated conductors 35 which extend through the said tubes to a common connection 37 leading to the other terminal of the filament potential source.

In order that each copper tube and its central conductor shall be maintained at the same radio frequency potential, a by-pass condenser Be is connected across each filament closely adjacent to the tubes. A tuning condenser ll may or may not be necessary depending upon the frequency to be handled by the amplifier but, if used, it is connected as shown in the drawing between the copper tubes at points close to the thermionic devices I and 3.

A connection 33 extends from the bridging member El to a terminal of the filament potential source, this connection preferably including. a grid leak 45 and a grid condenser ll for the purpose of maintaining the grids of the thermionic devices at the proper operating potential.

From the foregoing, it will be apparent that, insofar as radio frequencies are concerned, the copper tubes and the inner conductors constitute a single inductor which may be tuned either by the shunting condenser 4| or by movement of the bridging member 33. Obviously, a single copper tube formed into a helix could be utilized, if desirable. In such case the length of the tubing will be governed by the frequency desired, whose electrical centre is the nodal point for entrance of connection 31.

The pair of conductors constituted by the copper tubes and the adjustable bridge therebetween is tuned by the bridging condenser M to form a resonant circuit. It is apparent that, if the bridging member 33 were placed closely adjacent to the filaments, and if the grid and plate circuits were tuned to the same period, self-oscillation would take place by reason of the feed-back through the capacity of the several tubes. If, however, the bridging member is properly located along the copper tubes and the tuning condenser adjusted, self-oscillation can be suppressed. The explanation of this fact is as follows:

In order for a tube to oscillate, the voltage fed back through the tube capacitance from the plate to the grid must be opposite in sign to the instantaneous voltage appearing upon the plate, i. e., when the plate is swinging positive the grid must be displaced substantially electrical degrees negative. It can be seen that, if the filament circuit is properly tuned, and on account of the coupling between the elements, radio frequency potentials will be developed on the filaments themselves. This voltage can be so phased as to produce a voltage having a sign like that of the plate, at any instant, and, therefore, it will be impossible to produce self-oscillation, for the reason that a balance is obtained equivalent to that of a bridge circuit. It can also be seen, from a consideration of the foregoing, that the net potential of the grid can be so adjusted that feed-back is impossible for the reason that the filament, in eifect, is the same as the screen in a screen grid tube.

My invention may also be practiced if the thermionic devices are of the equipotential type as shown in Fig. 2. In such case, a tunable circuit constituted by an inductor d9, shunted by a tuning condenser 55, is interposed between the cathodes. If the amplifier stage is to handle ultra high frequencies, the inductor may be constituted by a plurality of straight inductors and a bridgll'lg member analogous to the bridging member 33 shown in Fig. 1.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In an amplifier for ultra-high frequencies, a plurality of thermionic devices, a resonant input and a resonant output circuit connected to said thermionic devices in push-pull relation, a circuit interconnecting the cathodes of said devices and means to adjust the electrical length of said circuit to secure an in-phase relation of voltages impressed across the cathodes and the plates of said devices so as to neutralize the grid-toplate and grid-to-cathode capacitances inherent in said devices and thereby to avoid feed back.

2. In an amplifier for ultra-high radio frequencies, a plurality of thermionic tubes having grid and anode electrodes connected in push-pull relation to an input circuit and an output circuit substantially resonant to the same frequency, cathode potential supply connections extending between the cathodes of the said devices, and means for causing a standing wave to appear on said connections, and for rotating the instantaneous phase relations between the potentials appearing on the anodes and the cathodes of the said devices to secure an in-phase relation between said potentials so as to prevent the generation of spurious oscillations.

3. In an amplifier for ultra-high radio frequencies, a plurality of thermionic devices connected in push-pull relation, each of said devices having a cathode, corresponding cathode terminals being interconnected by a hollow conducting tube, the remaining terminals of the. cathodes of said devices being connected through a conductor extending through the said hollow tube and insulated therefrom, means for maintaining the tube and conductor at the same potential with respect to radio frequencies, and means for so tuning the composite. conducting channel constituted by the tube and the conductor that instantaneous ultra high frequency potentials of like sign will at all times be impressed upon the anode and upon the grid of either thermionic device, said potentials being oppositely phased in any two of said devices which are connected in push-pull relation.

4.. In an amplifier, a pair of thermionic devices each having a filament cathode and at least two other electrodes, a resonant input circuit and a resonant output circuit connected in pushpull relation to said thermionic devices, concentric conductors to the cathode of each thermionic device, and means including a tuning bridge adjustably disposed in contact with the outer ones of said concentric conductors to secure an inphase relation as between potentials traversing the two said cathodes so as to substantially neutralize the grid-to-plate and grid-to-cathode capacitances inherent in said devices whereby to prevent the generation of spurious oscillations.

5. In a push-pull amplifier, a pair of thermionic devices each having a filament cathode and at least two other electrodes, an input circuit and an output circuit resonant at substantially the same frequency and connected in push-pull relation to said thermionic devices, means including concentrically disposed conductors for supplying heating current to said filament cathodes, and means including an adjustable bridge disposed in contact with the external ones of said conductors or mi to secure an in-phase relation as between potentials applied to said other electrodes so as to prevent feed-back through interelectrode capacity of energy from the output to the input circuit of each device.

6. In an amplifier, a pair of thermionic devices having filament cathodes, a resonant input circuit and a resonant output circuit connected in push-pull relation to said thermionic devices, electrically parallel conductors, one to a terminal of each cathode, electrically parallel tubular conductors, one to the remaining terminal of each cathode, one of the first said conductors in each case being disposed internally of one of said tubular conductors, and means comprising a tuning bridge adjustably disposed in contact with said tubular conductors to secure an in-phase relation between the potentials across other electrodes of said thermionic devices so as to prevent the generation of spurious oscillations.

7. In an amplifier for ultra high frequencies, a plurality of thermionic devices each having a plate, a grid and a cathode, a tunable circuit interconnecting said plates, a tunable circuit interconnecting said grids, a third circuit interconmeeting said cathodes, a connection from said plate circuit and from said grid circuit to said cathode circuit, and means to adjust the electrical length of said cathode circuit to secure an in-phase relation of voltages impressed across the cathodes and plates of said devices to neutralize the interelectrode capacitance and thereby to prevent the transfer of energy from said plate circuit through said tubes to said grid circuit.

JOHN EVANS. 

